1. Field of the Invention
The present invention relates to the papermaking arts. More specifically, the present invention relates to forming fabrics for the forming section of a paper machine.
2. Description of the Prior Art
During the papermaking process, a cellulosic fibrous web is formed by depositing a fibrous slurry, that is, an aqueous dispersion of cellulose fibers, onto a moving forming fabric in the forming section of a paper machine. A large amount of water is drained from the slurry through the forming fabric, leaving the cellulosic fibrous web on the surface of the forming fabric.
The newly formed cellulosic fibrous web proceeds from the forming section to a press section, which includes a series of press nips. The cellulosic fibrous web passes through the press nips supported by a press fabric, or, as is often the case, between two such press fabrics. In the press nips, the cellulosic fibrous web is subjected to compressive forces which squeeze water therefrom, and which adhere the cellulosic fibers in the web to one another to turn the cellulosic fibrous web into a paper sheet. The water is accepted by the press fabric or fabrics and, ideally, does not return to the paper sheet.
The paper sheet finally proceeds to a dryer section, which includes at least one series of rotatable dryer drums or cylinders, which are internally heated by steam. The newly formed paper sheet is directed in a serpentine path sequentially around each in the series of drums by a dryer fabric, which holds the paper sheet closely against the surfaces of the drums. The heated drums reduce the water content of the paper sheet to a desirable level through evaporation.
It should be appreciated that the forming, press and dryer fabrics all take the form of endless loops on the paper machine and function in the manner of conveyors. It should further be appreciated that paper manufacture is a continuous process which proceeds at considerable speeds. That is to say, the fibrous slurry is continuously deposited onto the forming fabric in the forming section, while a newly manufactured paper sheet is continuously wound onto rolls after it exits from the dryer section.
Woven fabrics take many different forms. For example, they may be woven endless, or flat woven and subsequently rendered into endless form with a seam.
The present invention relates specifically to the forming fabrics used in the forming section. Forming fabrics play a critical role during the paper manufacturing process. One of its functions, as implied above, is to form and convey the paper product being manufactured to the press section.
However, forming fabrics also need to address water removal and sheet formation issues. That is, forming fabrics are designed to allow water to pass through (i.e. control the rate of drainage) while at the same time prevent fiber and other solids from passing through with the water. If drainage occurs too rapidly or too slowly, the sheet quality and machine efficiency suffers. To control drainage, the space within the forming fabric for the water to drain, commonly referred to as void volume, must be properly designed.
Contemporary forming fabrics are produced in a wide variety of styles designed to meet the requirements of the paper machines on which they are installed for the paper grades being manufactured. Generally, they comprise a base fabric usually woven from monofilaments and may be single-layered or multi-layered. The yarns are typically extruded from any one of several synthetic polymeric resins, such as polyamide and polyester resins, used for this purpose by those of ordinary skill in the paper machine clothing arts.
The design of forming fabrics additionally involves a compromise between the desired fiber support and fabric stability. A fine mesh fabric may provide the desired paper surface properties, but such design may lack the desired stability resulting in a short fabric life. By contrast, coarse mesh fabrics provide stability and long life at the expense of fiber support. To minimize the design tradeoff and optimize both support and stability, multi-layer fabrics were developed. For example, in double and triple layer fabrics, the forming side is designed for support while the wear side is designed for stability and drainage.
In addition, triple layer designs allow the forming surface of the fabric to be woven independently of the wear surface. Because of this independence, triple layer designs can provide a high level of fiber support and an optimum internal void volume. Thus, triple layers may provide significant improvement in drainage over single and double layer designs.
Essentially, triple layer fabrics consist of two fabrics, the forming layer and the wear layer, held together by binding yarns. The binding is extremely important to the overall integrity of the fabric. One problem with triple layer fabrics has been relative slippage between the two layers which breaks down the fabric over time. In addition, the binding yarns can disrupt the structure of the forming layer resulting in marking of the paper. See e.g., Osterberg (U.S. Pat. No. 4,501,303), the contents of which are incorporated herein by reference. In order to further improve the integrity of the fabric and sheet support, triple layer fabrics were created incorporating binder pairs. These pairs of binders are incorporated into the structure in a variety of weave patterns and picking sequences. See e.g., Seabrook et al. (U.S. Pat. No. 5,826,627) and Ward (U.S. Pat. No. 5,967,195), the contents of which are incorporated herein by reference.
As mentioned above, the fabric is installed as a continuous belt which is rotated through the papermaking machine at considerable speeds. It is important to have a forming fabric with good CD stability to provide acceptable sheet profiles as the operating speeds of papermaking machines increase. This has been accomplished in the prior art with the triple stacked shute (TSS) concept. TSS fabrics add a CD yarn stacked between the forming side shute and the wear side shute to acts as a CD stabilizer. Several closely related patents exist covering triple stacked shute (TSS) designs; e.g. JP 6-4953, U.S. Pat. No. 4,379,735, U.S. Pat. No. 4,941,514, U.S. Pat. No. 5,164,249, U.S. Pat. No. 5,169,709 and U.S. Pat. No. 5,366,798, the contents of which are incorporated herein by reference.
One disadvantage of the TSS concept is that the design adds additional caliper and void volume to the fabric which adversely impacts the efficiency of the vacuum elements in the papermaking machine. This is because the vacuum elements have to rid the fabric voids of water before they start de-watering the paper sheet. Thus, a need exists for a forming fabric that reduces the void volume of the fabric without increasing the caliper.
Furthermore, it is desired that multi-layer fabrics have more cross-directional stability and stiffness to prevent cross directional shrinkage and improve sheet formation and appearance.
The present invention is a multi-layer forming fabric having CD packing yarns added to the wear-side layer. The packing yarns add to the CD stability of the fabric and greatly reduce the void volume without adding to the fabric caliper and not significantly disrupting the air permeability of the fabric.